1) Field of the Invention
The present invention relates to a method for intra cell handover and channel allocation in cellular phone systems, and specifically to a method for reducing interference with other users by handoff of a call causing interference.
2) Discussion of Related Art
In cellular telephone systems, a radio link, i.e., a call, between a mobile station and a base station may be switched to a different channel when prevailing reception conditions make this handoff procedure advantageous. The main handover strategies existing in cellular networks are handover between two different cells, i.e., inter cell handover, and handover within a cell, i.e., intra cell handover. Inter cell handover is made when the path loss between a mobile station (MS) and a neighboring base station (BS) is less than the path loss to the base station it is presently in communication with. Intra cell handover is made when a carrier to interference ratio (C/I) is below a certain limit at the same time the path loss in the cell that the mobile is in communication with is less than another limit. These rules can be expressed as:
Inter cell handover: Path loss between MS and a neighboring BS &lt; Path loss between MS and own MS - hysteresis. Intra cell handover: C/I in uplink or C/I in downlink &lt; C/I limit AND Path loss &lt; path loss limit.
The hysteresis in inter cell handover is needed to avoid repeated handovers at cell borders.
In the TDMA system based on the standard EIA/TIA--IS-54, power is transmitted on all time slots on a carrier in the downlink if at least one time slot is in use. However, the C/I ratio is only checked in the time slot that is currently in use.
In this system, a problem has arisen as follows. The quality, or C/I ratio, in a link depends on both the signal strength C and the interference level I. Both are equally important. When the call suffers from poor quality, a handover should be made in order to find a better channel. A handover order is sent from the base station BS to the mobile station MS and the mobile station MS has to receive the order before it can tune to a new channel. The problem is that the reason for the handover attempt may have been poor quality in the downlink, and thereby the handover order may not be received by the mobile station MS. The call will be lost if the interference in the downlink does not decrease.
The goal for intra cell handover is to find a channel which suffers from less interference in order to improve the C/I ratio. The reason for high interference in the uplink is that some other call is currently using the same channel. The other call may be on the same channel causing interference because it is in communication with a neighboring cell on the same channel or because of misalignment of the time slot due to propagation delay, for example. In the downlink, interference is caused by at least one other call linked to a different cell currently using the same carrier. Even though the quality may be high in both links for a call, the call may severely interfere with another call in the downlink. It is equally important for a user not to interfere with another user as it is for the user not to be subject to interference, but the former cannot be avoided in current intra cell handover criteria.